Stimulation of cardiac myoblast proliferation offers a possible means to replace heart cells lost as a result of ischemic heart diseases. Accordingly, we are studying the role of anabolic hormones and growth factors in the growth and division of muscle cells in culture. We have found that Somatomedin A gives substantial stimulation of myoblast proliferation, and we now propose to characterize the actions of MSA (Temin's Multiplication Stimulating Activity), a close analog used as a somatomedin surrogate in our experiments, on myoblasts in culture. Our initial experiments indicate that some aspects of this system differ significantly from those described for most peptide hormones. The proposed research is designed to evaluate the hypothesis that different receptors or different hormone-receptor interaction times are involved in the mitogenic (as distinguished from transport or metabolic) actions of peptide hormones and related growth factors. On the basis of observations of the time course and modulation of binding of these peptides, we suggest that initial peptide-receptor interactions are involved in early effects, but not in subsequent mitogenic actions, and that the use of 125I-labeled hormones may give misleading results in long-term experiments because of preferential metabolism of these modified peptides. We will evaluate our hypotheses by studying binding and actions of MSA on Yaffe's L6 cell line; this system offers a number of technical and some theoretical advantages. We will exploit the unique aspects of MSA production by a liver cell line to prepare 3H-MSA for the essential evaluation of the validity of 125I-MSA in studies of long-term binding of MSA to muscle cells. We will then compare relationships of binding to transport and mitogenic effects of MSA as affected by incubation time and by various modulators such as insulin, cytochalasins, and corticosteroids.